1. Field of the Invention
The invention relates to a method of proximity exposure by oblique irradiation of a workpiece with UV radiation. The invention relates especially to a proximity exposure method which is suited for optical alignment of an alignment layer of a LCD element.
2. Description of Related Art
To determine the size of the pre-tilt angle and the direction of alignment of a liquid crystal of an LCD element, a device was proposed in which an alignment layer, as the workpiece, is obliquely irradiated with UV radiation; see, for example, paragraphs (0011) and (0086) of published Japanese Patent Application HEI 9-197409 (also UK application GB 2310934 B1, and German application 19654638).
The angle of incidence and the irradiation angle of the light incident on the workpiece are different depending on the type of alignment layer and other treatment conditions.
In the alignment of the alignment layer of an LCD element, a pixel is divided into two or more pixels and the alignment direction of the liquid crystals is changed for the respective pixel which is formed by the division, by which the angle of view field of the liquid crystal display is improved. This method is called the pixel division method or multi-domain method.
In the case of using the optical alignment for this pixel division method, part of the pixel formed by division is irradiated with UV light using a mask. Next, the mask is changed or is turned horizontally relative to the workpiece. The other part of the pixel which is formed by division is irradiated from a direction which differs from the first irradiation direction, proceeding from the workpiece. Under certain circumstances, the irradiation angle is changed. By repeating this process as frequently as the number of divisions, the alignment direction of the liquid crystals can be changed for the respective pixel which is formed by division. In this case, only the desired parts may be exactly irradiated with UV radiation via the mask. For this purpose, the mask pattern must be positioned relative to the desired UV radiation and relative to the area of the workpiece to be irradiated, and irradiation is performed with parallel light.
FIG. 7 schematically shows LCD elements. As the drawings show, there are a host of LCD elements on the substrate, each group being formed from three pixels P1, P2 and P3 which correspond to R(red), G (green) and B (blue) respectively. Pixels P1, P2 and P3 each have a length of roughly 150 microns and a width from roughly 50 to 100 microns. Between each of the pixels, there is a zone which is called the xe2x80x9cblack matrixxe2x80x9d and which does not transmit light. In the case of a liquid crystal display of the TFT (thin film transistor) drive type, the respective pixel P1 to P3 is partially provided with a drive TFT element.
In a pixel division method as shown in FIG. 7, each pixel is divided into several pixels. In the respective zone which is formed by division, the direction of the alignment of the liquid crystals (in the drawing, the arrow directions in pixel P2) is changed. On the boundary line of the respective zone which is formed by the division, light is transmitted by scattering of the direction of alignment, and irregularities form in the images. To prevent this, there is a black matrix with a width of from 10 to 20 microns. When the width of this black matrix becomes larger, the degree of opening of the pixel becomes smaller, by which it becomes darker. The narrower the black matrix, the better. The width currently desired is 5 microns. In FIG. 7, only the pixel P2 is divided. However, in fact, all of the pixels P1 to P3 are divided in the same way as in P2.
Light irradiation for optical alignment is conventionally performed as follows:
Here, a case is described in which a pixel is divided into four pixels, i.e., into first through fourth zones, and optical alignment is performed. In the pixel P2 as shown in FIG. 7, for example, the zone at the top right is called the first zone, the zone at bottom right is called the second zone, the zone at bottom left the third zone and the zone at top left the fourth zone.
(1) FIG. 8 schematically shows one example of a mask M which is used for optical alignment. As is shown in the drawings, in the mask M, at the sites which each correspond to the first zone of the pixel formed by division, there are openings OP (in the drawings, the area surrounded by the broken line corresponds to a pixel). Furthermore, on the mask M, alignment marks MAM for positioning are recorded.
Using the mask M which is shown in the drawing, the first zones of the workpiece are irradiated with light via the mask M with a predetermined angle of incidence with reference to the mask and a preset irradiation angle with respect to the X-direction of the mask pattern.
(2) Next, the workpiece is turned, for example, by 180xc2x0 and the third zones which are arranged point-symmetrically around the pixel center are irradiated with light with a predetermined angle of incidence and a preset irradiation angle.
(3) Then, the mask is replaced with another mask which has openings which correspond to the second zones and which differ from the first zones. The third zones are irradiated with light with a predetermined angle of incidence and a preset irradiation angle.
(4) The workpiece is turned by 180xc2x0. The fourth zones which are arranged point-symmetrically around the pixel center relative to the second zones are irradiated with light with a predetermined angle of incidence and a preset irradiation angle.
When, in optical alignment of the respective zones of the pixel, the adjacent zones are irradiated with this light, scattering of the alignment and thus irregularities of the images occur. Therefore, it is necessary for the deviation in the positioning of the mask relative to the workpiece to be smaller than the value of the width of the black matrix located around the pixel (for example, preferably 5 microns, as described above).
For LCD elements, large substrates having dimensions of 550xc3x97650 mm to 650xc3x97830 are becoming more and more frequent and important. To treat large substrates with a high throughput in this way, therefore proximity exposure is used as the exposure process in which a mask and a workpiece are brought into proximity with one another until a predetermined spacing is reached (the mask and the workpiece do not come into contact with one another), and in which the entire substrate is irradiated overall with parallel light.
In the case of a conventional proximity exposure device, a horizontally arranged workpiece is irradiated vertically with light. Here, the optical axis of the irradiated light (parallel light) and the mask are arranged perpendicular to one another.
The mask and the workpiece are positioned as follows:
Using an alignment microscope in which the optical axis of the alignment light is located parallel to the optical axis of the irradiation light, the alignment marks recorded on the mask and the alignment marks recorded on the workpiece are determined and the mask or the workpiece is moved such that they come to rest on one another. The mask is positioned relative to the workpiece with respect to the vertical direction. After completion of positioning, the workpiece is irradiated via the mask with light. The irradiation light, as parallel light, is incident vertically on the mask. The mask pattern is projected into a stipulated area of the workpiece and shields the covered area. The UV radiation is emitted only onto a desired area.
When, in a proximity exposure device, an attempt is made to irradiate the alignment layer workpiece obliquely with UV radiation, in order to determine the size of the pre-tilt angle and the direction of alignment of the liquid crystals, as described above, the light must be allowed to be incident obliquely on the mask.
When the mask is irradiated obliquely with light, however, on the workpiece, a position deviation of the mask pattern which has been projected by the irradiation light is formed because, in proximity exposure, there is a gap between the mask and the workpiece.
In a conventional proximity exposure method, the positioning of the mask relative to the workpiece with respect to the vertical direction is performed. When the mask is irradiated obliquely with light, the location at which the mask pattern is projected onto the workpiece deviates. The irradiation light is thus emitted outside the desired area.
The invention was devised to eliminate the above described defect in the prior art. Therefore, a primary object of the present invention is to devise a proximity exposure method in which the mask pattern can be projected onto a stipulated area of the workpiece so that only a desired area of the workpiece is irradiated with light, even if the light is allowed to be incident obliquely on the mask.
The object is achieved in accordance with the invention as follows:
(1) In a proximity exposure method, in which a mask on which a mask pattern is formed has a predetermined spacing relative to a workpiece, and in which the workpiece is irradiated obliquely with light via the mask so that the mask pattern is exposed onto the workpiece, positioning of the workpiece relative to the mask is performed based on the angle of incidence and the irradiation angle of the light with respect to the mask, and based on the above described spacing, and the mask pattern is exposed onto only a desired area of the workpiece.
(2) In a proximity exposure method in which a mask on which a mask pattern is formed has a predetermined spacing relative to a workpiece, and in which the workpiece is irradiated obliquely with light via the mask and the mask pattern is exposed onto the workpiece, the amount of deviation between the projection site of the mask pattern on the workpiece in vertical irradiation of the mask with light and the projection site of the mask pattern on the workpiece with oblique irradiation of the mask with light is computed based on the angle of incidence and the irradiation angle of the light with respect to the mask, and based on the size of the spacing between the mask and the workpiece, positioning of the workpiece to the mask is preformed based on this computed amount of deviation and the mask pattern is exposed onto the workpiece.
(3) In a proximity exposure method, using a proximity exposure device which comprises:
an alignment microscope for determining the mask alignment marks and the workpiece alignment marks;
a gap measuring device for measuring the gap between the mask and the workpiece;
a workpiece carrier movement device for moving a workpiece carrier in the X-Y-Z-"THgr" directions; and
a light irradiation device for oblique light irradiation of the workpiece via the mask, the following steps are performed:
the workpiece is arranged relative to the mask with a predetermined gap therebetween;
using the gap measuring device, the size of the gap between the mask and the workpiece is measured;
viewing from a direction perpendicular to the mask, determining position coordinates (Xn, Yn) of the mask alignment marks and of the workpiece alignment marks by means of the alignment microscope;
by moving the workpiece carrier in the X-Y-"THgr" directions, the position coordinates (Xn, Yn) of the workpiece alignment marks are moved to the position coordinates (X, Y) which are designated with the formulas described below, when for any X-Y coordinates, the determined position coordinates of the workpiece alignment marks are (Xn, Yn) and the position coordinates of the workpiece alignment marks for positioning of the workpiece alignment marks relative to the mask alignment marks by the movement of the workpiece carrier in the X-Y-"THgr") directions are (Xn+xcex94X, Yn+xcex94Y);
the mask and the workpiece are positioned to one another; and
the mask pattern is exposed onto the workpiece.
X=Xn+xcex94Xxe2x88x92Gxc2x7tan xcex4xc2x7cos "PHgr"
Y=Yn+xcex94Yxe2x88x92Gxc2x7tan xcex4xc2x7sin "PHgr"
where G is the size of the gap between the mask and the workpiece, xcex4 is the angle of incidence of the irradiation light into the mask and "PHgr" is the irradiation angle of the irradiation light which is incident in the X-direction of the mask pattern.
As was described above, according to the invention, based on the angle of light incidence xcex4, the irradiation angle "PHgr" and the size of the gap G, positioning of the mask relative to the workpiece is performed and the pattern is exposed onto the workpiece. In this way, the mask pattern can be projected onto a predetermined position of the workpiece and the desired area can be irradiated with light.
In the following, the invention is described specifically based on the embodiment shown in the drawing.